Reflection infrared spectra analysis

A method of horizontal attenuated total reflectance (HATR) by Fourier transform infrared spectroscopy (FTIR) is considered more useful for infrared spectroscopic analysis of leather samples. By this method, the sample is simply put on the flat plate of ZnSe crystal or KRS-5 crystal of the HATR accessory. The Fourier transformation of the interferogram is converted by the computer into a plot of absorption against wave number that resembles the usual IR spectrum (William and Fleming, 1998). 

Diffuse reflectance or DRIFTS (diffuse reflectance infrared Fourier-lransform spectroscopy) allows the sain)le to be analysed neat, ot diluted in a non-absorbing matrix (e.g. KCl or KBr at 1-5% w/w analyte). DRIFTS also may be used to obtain the spectrum of a solute in a volatile solvent by evaporating the solution onto KBr. When the IR radiation interacts with the powdered sample it will be absorbed, reflected and diffracted. The radiation which has been diffusely reflected contains vibrational information on the molecule. This technique allows non-destructive testing of neat materials and is suited to quantitative analysis, although care must be taken to ensure that a consistent particle size is used. 

The primary components and the chemical structure of the raw peat and the solid product were further analyzed by Fourier transform infrared spectroscopy (FTIR) 0ASCO 670 Plus) using the Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) technique and the JASCO IR Mentor Pro 6.5 software for spectral analysis. The cross polarization/magic angle spinning (CP/MAS) NMR spectrum of raw peat and the solid... 

To conclude this article, it is important to state that, in general, commercial oenological laboratories are equipped with automated instrumentation that carry out the above analyses (and others besides) in a single step. The most widely used instrumental technique is based on FTIR analysis. The infrared spectrum of an organic solution such as wine presents complex absorption spectra characteristic of the different wine components. The Michelson interferometer, which is at heart of the FTIR method, is based on the division of a polychromatic band of infrared radiation into two beams which then follow different optical pathways one beam traverses the sample cell directly, while the other is reflected on a mobile mirror before arriving at the sample cell. For each elemental wavelength arriving at the detector cell there will be a phase difference, which is continuously varied... 

Figure 14.33 a) ATR spectra of single nylon carpet fiber untreated (top), treated (middle), and the result of spectral subtraction (bottom), b) Bicomponent fiber transmission spectrum (top), ATR spectrum of Nylon 6 sheath (middle), and difference spectrum of PET core (bottom).Reproduced with permission from Cho, L, et a ., "Single Fiber Analysis by Internal Reflection Infrared Microspectroscopy," iourna of Forensic Sciences 46 (2001), 1309-1314. Copyright 2001, ASTM International. 

Further, the techniques of Scanning Electron Microscopy (SEM), Specular Reflectance Infrared Spectroscopy (SRIS), and Electron Spectroscopy for Chemical Analysis (ESCA) have proved complementary in this investigation of the relationships between adherend surfaces and adhesive properties. As SRIS results have shown the presence of adhesive on all fracture samples, ESCA and SEM results further clarified the nature of the fracture surface through the presence or absence of a Ti ESCA spectrum and the observation or lack of observation of the substrate structure in the SEM photomicrographs. It is concluded from the results of the three techniques that for the Set I samples, cohesive failure was noted for 219D2 whereas adhesive failure was noted for 220D3. Cohesive failure was noted for samples lm2-517 and lmp2-516 and adhesive failure was noted for 2m2-515 in Set II. 

For qualitative analysis of a compound separated by TLC (thin-layer chromatography), it is a usual practice to extract the compound with a solvent from a spot in the chromatograph, and then examine it to identify its chemical stmcture by an appropriate analytical instrument such as a mass spectrometer, an FT-IR spectrometer, or an NMR spectrometer. In favorable circumstances, the diffuse-reflection measurement may provide an FT-IR spectrum of the compound without the troublesome process of solvent extraction. In Figure 12.9, the infrared spectrum of cholesterol acetate measured by the diffuse-reflection... 

Owing to these improvements, infrared spectroscopy has undergone a marked development in the last few years because of the possibility of adapting new accessories to the spectrometers and, therefore, of analysing samples whose infrared spectrum was impossible to obtain some years ago. Among these accessories are those which can measure specular and diffuse reflection, attenuated total reflectance and microscopes. One area in which the technique has improved dramatically is forensic analysis. 

Spots on papers can also be measured. Figure 7 shows the spectra of two samples from the same piece of paper, one of which is spotted. The attenuated total reflectance objective of the microscope allows us to identify the presence of paraffins. In such cases, a microscope is essential to obtain the infrared spectrum while the attenuated total reflectance objective allows the analysis to be carried out without destroying the document. 

The external reflection of infrared radiation can be used to characterize the thickness and orientation of adsorbates on metal surfaces. Buontempo and Rice [153-155] have recently extended this technique to molecules at dielectric surfaces, including Langmuir monolayers at the air-water interface. Analysis of the dichroic ratio, the ratio of reflectivity parallel to the plane of incidence (p-polarization) to that perpendicular to it (.r-polarization) allows evaluation of the molecular orientation in terms of a tilt angle and rotation around the backbone [153]. An example of the p-polarized reflection spectrum for stearyl alcohol is shown in Fig. IV-13. Unfortunately, quantitative analysis of the experimental measurements of the antisymmetric CH2 stretch for heneicosanol [153,155] stearly alcohol [154] and tetracosanoic [156] monolayers is made difflcult by the scatter in the IR peak heights. 

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